Japan scrambled eight fighter jets on Thursday after a Chinese state-owned plane breached its airspace for the first time, over islands at the centre of a dispute between the countries.
It was the first incursion by a Chinese state aircraft into Japanese airspace anywhere since Tokyo's military began monitoring in 1958, the defence ministry said.
The move marks a ramping-up of what observers suggest is a Chinese campaign to create a "new normal" -- where its forces come and go as they please around islands Beijing calls the Diaoyus, but Tokyo controls as the Senkakus.
It also comes as ceremonies mark the sensitive 75th anniversary of the start of the Nanjing Massacre, when Japanese Imperial Army troops embarked on an orgy of violence and killing in the then-Chinese capital.
F-15 jets were mobilised after a Chinese Maritime Surveillance twin turbo-prop aircraft ventured over the islands just after 11 am (0200 GMT), Chief Cabinet Secretary Osamu Fujimura told reporters.
"It was a fixed-wing Y-12 aeroplane belonging to the Chinese State Oceanic Administration. We confirmed that this aeroplane flew in our country's airspace," he said.
"It is extremely regrettable. We will continue to resolutely deal with any act violating our country's sovereignty, in accordance with domestic laws and regulations," he said, adding a senior Chinese diplomat had been summoned.
Japan mobilised eight F-15 jets and an E2C early-warning aircraft, the Asahi Shimbun reported, citing a defence ministry source. But the incident appeared to have passed off without any direct confrontation.
In Beijing, China's foreign ministry said the flight had been routine.
"China's maritime surveillance plane flying over the Diaoyu islands is completely normal," said spokesman Hong Lei.
"China requires the Japanese side to stop illegal activities in the waters and airspace of the Diaoyu islands," Hong said, adding they were "China's inherent territory since ancient times".
The incident came as Japan's coastguard chief told reporters he was digging in for a protracted dispute.
"As China has publicly said it will make this a permanent situation, we are preparing to be better equipped for this long, drawn-out contest," Takashi Kitamura, the commandant of Japan Coast Guard, told a news conference.
"Because we have various other responsibilities other than patrolling for border security, we are asking government to consider building up our capacity," he said.
Chinese government ships have moved in and out of waters around the islands for more than two months -- four vessels were there for several hours on Thursday.
Such confrontations have become commonplace since Japan nationalised the East China Sea islands in September, a move it insisted amounted to nothing more than a change of ownership of what was already Japanese territory.
But Beijing reacted with fury, with observers saying the riots that erupted across China had at least tacit backing from the Communist Party government.
Mitsuyuki Kagami, an expert in Chinese politics at Aichi University said there would be no let-up from Beijing.
"China will keep sending official ships and probably aeroplanes to undermine the status quo of Japan's control over the islands," he told AFP.
He said it would be more alarming if it began to send military vessels or aircraft, but he believed Beijing had no interest in a war with Tokyo.
"China hopes to draw Japan to the negotiating table," he said, adding that the likely victory of the hawkish Shinzo Abe in Japan's general election on Sunday might make any Japanese compromise more difficult.
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